Question

我有一个像这样的代码：

std::vector<int> v1 = { 1, 2, 3, 4 };  
std::vector<int> v2 = { 7, 8, 9, 10 };  
std::vector<int>::iterator it = std::next(v1.begin());  
v1 = v2;  
int test = *it;  
std::cout << test;

上面的代码会抛出一个错误：迭代器不能解除引用。

但是，如果我用列表替换vector如下：

std::list<int> v1 = { 1, 2, 3, 4 };  
std::list<int> v2 = { 7, 8, 9, 10 };  
std::list<int>::iterator it = std::next(v1.begin());  
v1 = v2;  
int test = *it;  
std::cout << test;

代码只是按预期运行而没有错误从Iterator invalidation rules和std::list::operator=开始，在调用operator =后，我被告知，除了最终迭代器之外，与此容器相关的所有迭代器，引用和指针都无效。但为什么上面的代码与std :: list有效？我误解了一些必要的东西吗？

Answer 1

当迭代器失效时，取消引用它是未定义的行为。因此，就规范而言，无论你采取何种行为都是无关紧要的。＆＃34;根据您的期望，工作＆＃34;是允许的行为之一。

FWIW（不多，tbh），我希望std::list赋值运算符的实现等同于：

list& operator=(list const& rhs) {
    if (this == &rhs)
        return *this;

    auto lhs_i = begin();
    auto rhs_i = rhs.begin();

    // write over the elements in any currently existing nodes, this
    // avoids any unnecessary allocations
    while (lhs_i != end() && rhs_i != rhs.end()) {
        *lhs_i++ = *rhs_i++;
    }

    // erase any extra elements if size() > rhs.size()
    erase(lhs_i, end());

    // push back additional elements if size() < rhs.size()
    while (rhs_i != rhs.end()) {
        push_back(*rhs_i++);
    }

    return *this;
}

如果是，你可以看到，对于像你这样的情况，列表都有相同数量的元素，没有创建或销毁任何元素，所以你非常希望迭代器继续工作完全正常。当然，这完全是猜测，绝对不是你应该依赖的行为，因为即使是实施的情况，他们也可以在下一个版本中更改它而不另行通知。

Answer 2

这是未定义的行为，但GCC有调试容器来捕获这种行为。

使用Attempt to reload List/Util.pm aborted. Compilation failed in require at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Sub/Exporter/Progressive.pm line 9. BEGIN failed--compilation aborted at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Sub/Exporter/Progressive.pm line 9. Compilation failed in require at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Devel/GlobalDestruction.pm line 11. BEGIN failed--compilation aborted at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Devel/GlobalDestruction.pm line 11. Compilation failed in require at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Moo/_Utils.pm line 21. BEGIN failed--compilation aborted at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Moo/_Utils.pm line 21. Compilation failed in require at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Moo.pm line 4. BEGIN failed--compilation aborted at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/Moo.pm line 4. Compilation failed in require at /home/travis/build/xtaran/CGI-Github-Webhook/build_dir/lib/CGI/Github/Webhook.pm line 11. BEGIN failed--compilation aborted at /home/travis/build/xtaran/CGI-Github-Webhook/build_dir/lib/CGI/Github/Webhook.pm line 11. Compilation failed in require at ./cgitest.pl line 10. BEGIN failed--compilation aborted at ./cgitest.pl line 10. script /cgi-bin/cgitest.pl generated no valid headers at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/CGI/Test.pm line 468. Use of uninitialized value $in_fname in unlink at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/CGI/Test.pm line 469. Use of uninitialized value $in_fname in concatenation (.) or string at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/CGI/Test.pm line 469. can't unlink : No such file or directory at /home/travis/perl5/perlbrew/perls/5.22.0/lib/site_perl/5.22.0/CGI/Test.pm line 469.：

启用它

-D_GLIBCXX_DEBUG

赋值（operator =）使容器的迭代器无效

2 个答案: